The present invention relates to the conversion of hydrocarbons such as through a Fischer-Tropsch reaction, and more particularly relates to hydrocarbon conversion process and system using a plurality of synthesis gas sources.
The synthetic production of hydrocarbons by the catalytic reaction of carbon monoxide and hydrogen is well known and is generally referred to as the Fischer-Tropsch reaction. The Fischer-Tropsch process was developed in early part of the 20th century in Germany. It has been practiced commercially in Germany during World War II and later in South Africa.
The Fischer-Tropsch reaction for converting synthesis gas (primarily CO and H2) has been characterized by the following general reaction: 
The hydrocarbon products derived from the Fischer-Tropsch reaction range from some methane to high molecular weight paraffinic waxes containing more than 100 carbon atoms.
Numerous catalysts have been used in carrying out the Fischer-Tropsch reaction. Usually a Group VIII metal, such as cobalt, iron, or ruthenium, is used. Both saturated and unsaturated hydrocarbons can be produced. The synthesis reaction is very exothermic and temperature sensitive whereby temperature control is required to maintain a desired hydrocarbon product selectivity.
While the Fischer-Tropsch process has been around for nearly eighty years, improved performance remains a goal. In particular, an ongoing quest exists to improve the economics of the process.
Synthesis gas (xe2x80x9csyngasxe2x80x9d), which is substantially carbon monoxide and molecular hydrogen, may be made from natural gas, gasified coal, and other sources. Three basic methods have been employed for producing synthesis gas utilized as feedstock in the Fischer-Tropsch reaction. Traditional methods include steam reforming, wherein one or more light hydrocarbons such as methane are reacted with steam over a catalyst to form carbon monoxide and hydrogen, and partial oxidation, wherein one or more light hydrocarbons are combusted sub-stoichiometrically to produce synthesis gas. The steam reforming reaction is endothermic and a catalyst containing nickel is often utilized. Partial oxidation is the catalytic or non-catalytic, sub-stoichiometric combustion of light hydrocarbons such as methane to produce the synthesis gas. The partial oxidation reaction is typically carried out using high-purity oxygen. High-purity oxygen, however, can be quite expensive and dangerous to handle.
In some situations these synthesis gas production methods may be combined to form a third method. A combination of partial oxidation and steam reforming, known as autothermal reforming, and which uses air (or O2) as a source of oxygen for the partial oxidation reaction, has also been used for producing synthesis gas heretofore. With autothermal reforming, the exothermic heat of the partial oxidation supplies the necessary heat for the endothermic steam reforming reaction. The autothermal reforming process can be carried out in a relatively inexpensive refractory lined carbon steel vessel.
The autothermal process results in a lower hydrogen-to-carbon-monoxide ratio in the synthesis gas than does steam reforming alone. That is, the steam reforming reaction with methane results in a ratio of about 3:1 or higher while the partial oxidation of methane results in a ratio of approximately 2:1. A good ratio for the Fischer-Tropsch (F-T) hydrocarbon synthesis reaction carried out at low or medium pressure (i.e. in the range of about atmospheric to 500 psig) over a cobalt catalyst is about 2:1. When the feed to the autothermal reforming process is a mixture of light shorter-chain hydrocarbons such as a natural gas stream, some form of additional control is desirable to maintain the ratio of hydrogen to carbon monoxide in the synthesis gas at the desired ratio, which for cobalt based F-T catalysts is about 2:1. Steam and/or CO2 may be added to the synthesis gas reactor to adjust the ratio.
Fischer-Tropsch hydrocarbon conversion systems typically have a synthesis gas generator or source as discussed above. The synthesis gas generator receives light, short-chain hydrocarbons such as methane and produces synthesis gas. The synthesis gas is then delivered to a Fischer-Tropsch reactor. In the Fischer-Tropsch reactor, the synthesis gas is converted to heavier, longer-chain hydrocarbons. Hundreds of example systems are shown in the literature; for example, U.S. Pat. Nos. 4,833,170 and 4,973,453, which are incorporated by reference herein for all purposes, present useful conversion systems.
It has been a quest for many to improve the economics of processes utilizing the Fischer-Tropsch reaction. Improved economics will allow for wide-scale adoption of the process in numerous sites and for numerous applications. Efforts have been made to improve economics, but further improvements are desirable.
A need has arisen for a system and method that addresses shortcomings of prior systems and methods. According to an aspect of the present invention, a process for converting light hydrocarbons to heavier hydrocarbons includes steps of: preparing a first synthesis gas having a H2:CO ratio greater than 2:1; removing a portion of the hydrogen from the first synthesis gas; preparing a second synthesis gas with a CO2 recycle wherein the second synthesis gas has a H2:CO ratio less than 2:1; adding the removed hydrogen to the second synthesis gas to increase the H2:CO ratio of the second synthesis gas; and using a Fischer-Tropsch reaction to convert the first synthesis gas and the second synthesis gas to heavier hydrocarbons. According to another aspect of the present invention, a first tail gas is also prepared in the first synthesis unit and is used in the second synthesis gas unit as a fuel. According to another aspect of the present invention, the second synthesis unit also prepares a second tail gas from which CO2 is removed and recycled to the second synthesis gas unit.
According to another aspect of the present invention, a system for converting light hydrocarbons into heavier hydrocarbons includes a first synthesis gas unit, which preferably has a steam methane reformer, for producing a first synthesis gas; a hydrogen separator coupled to the first synthesis gas unit for removing at least a portion of the hydrogen from a first synthesis gas to make a hydrogen-reduced synthesis gas; a second synthesis gas unit, which preferably has an autothermal reformer, for receiving an oxygen-containing gas, light hydrocarbons, and carbon dioxide and producing a second synthesis gas; a first synthesis unit fluidly coupled to the hydrogen separator for receiving the hydrogen-reduced synthesis gas and producing heavier hydrocarbons; a second synthesis unit fluidly coupled to the second synthesis gas unit and hydrogen separator for receiving a second synthesis gas from the second synthesis gas unit and hydrogen from the hydrogen separator unit and producing heavier hydrocarbons; and a carbon dioxide removal unit coupled to the second synthesis unit for receiving the tail gas therefrom and removing carbon dioxide therefrom and delivering the carbon dioxide to the second synthesis gas unit. According to another aspect of the present invention, the first synthesis unit is also operable to produce a first tail gas that may be used in the second synthesis gas unit. According to another aspect of the present invention, the second synthesis unit is operable to produce a second tail gas that may be used as a burner fuel in the first synthesis gas unit.
The present invention provides many advantages. A number of examples follow. An advantage of the present invention is that the system and method require less light hydrocarbons to produce a given quantity of product, i.e., it has a higher carbon efficiency. Another advantage of the present invention is that an autothermal reformer may be utilized at high pressure thereby allowing the removal of a synthesis gas booster compressor but without suffering a loss in carbon efficiency for the higher pressure. With respect to this advantage, the carbon efficiency of the autothermal reformer is reduced at higher pressure, but since CO2, which is produced at the higher pressure, is recycled, the effective efficiency is not reduced by increasing pressure.